Ion implanters are used in the manufacture of semiconductor devices and other materials. In such ion implanters, semiconductor wafers or other substrates are modified by implanting atoms of a desired species into the body of the wafer, for example to form regions of varying conductivity.
Ion implanters are well known and generally conform to a common design as follows. An ion source generally comprises an arc chamber in which a hot plasma is generated. The plasma will contain ions of a desired species to be implanted.
An extraction lens assembly produces an electric field that extracts ions from the ion source and forms a mixed beam of ions. Only ions of a particular species are usually required for implantation in a wafer or other substrate, for example a particular dopant for implantation in a semiconductor wafer. The required ions are selected from the mixed ion beam that emerges from the ion source by using a mass analysing magnet in association with a mass-resolving slit. By setting appropriate operational parameters on the mass-analysing magnet and the ion optics associated therewith, an ion beam containing almost exclusively the required ion species emerges from the mass-resolving slit. The ions travel along a flight tube as they pass through the mass-analysing magnet.
The ion beam is transported along a beam line to a process chamber where the ion beam is incident on a substrate held in place in the ion beam path by a substrate holder. The substrate may be a semiconductor wafer.
The various parts of the ion implanter are operated under the management of a controller, typically a suitably trained person, a programmed computer, or the like. A more detailed description of an ion implanter of this general type can be found in U.S. Pat. No. 4,754,200.
Ions may strike some components within the ion implanter relatively frequently (other than the substrate to be implanted). For example, ions with a large mass-to-charge ratio will not be deflected sufficiently by the mass-resolving magnet to pass through the mass-resolving slit. As a result, a beam dump may be provided to adsorb such ions. These ions striking the beam dump may cause sputtering of material. Care must be taken though, as material sputtered from the beam dump may become entrained within the ion beam and so contaminate the substrate.
In addition, there are times when the ion beam may be dumped into the beam dump on purpose. For example, instability in the ion beam may require that implantation of a wafer be stopped as quickly as possible. One way of achieving this is to switch off the mass-analysing magnet. With the magnet switched off, the ions merely follow a straight path rather than the usual curved path through the flight tube. The beam dump is positioned to absorb the ion beam when it is dumped in this way. Such a beam strike of the whole beam is likely to sputter more material. Although the material can no longer become entrained within the ion beam, there remains a problem in that the beam dump often has line of sight to the substrate. Consequently, material sputtered from the beam dump may still contaminate the substrate.
A further example of a component that frequently sees beam strike is the beam stop that resides downstream of the substrate. The ion beam may strike the beam stop when the substrate is moved away from the ion beam path, e.g. during mechanical scanning of the wafer during implants with a spot beam.
Unwanted material that has been sputtered from components such as a beam dump may travel to the substrate and subsequently the material may strike the substrate causing contamination or even damage to the devices being formed on the substrate. Moreover, sputtered material may adhere to another surface within the ion implanter. Surfaces adjacent to the ion beam are the most prone to receiving such deposits. As the amount of material deposited accumulates, the chances of the deposits delaminating to form flakes or particles increases. These flakes or particles frequently detach from their host surface and may become entrained in the ion beam. As a result, the flakes or particles contain sputtered material that still ultimately reaches the substrate.